[GHC] #873: Add System.Directory.installFile

[GHC]

#873: Add System.Directory.installFile
-------------------------------+--------------------------------------------
    Reporter:  simonmar        |        Owner:             
        Type:  task            |       Status:  new        
    Priority:  normal          |    Milestone:  6.6        
   Component:  libraries/base  |      Version:  6.4.2      
    Severity:  normal          |     Keywords:             
          Os:  Unknown         |   Difficulty:  Easy (1 hr)
Architecture:  Unknown         |  
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{{{System.Directory.copyFile}}} doesn't remove the target first (see notes
 in System/Directory.hs).  We should have an alternative version that
 ''does'' remove the target first, perhaps called {{{installFile}}}, and
 use this in Cabal when installing.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/873>
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